Protect Yourself From Tech Support Scams

Tech support scams are an industry-wide issue where scammers trick you into paying for unnecessary technical support services. Additionally, some scammers may try to identify themselves as a Microsoft MVP.

Removing and reconnecting the external drive didn't help. I've started chkdsk on the external drive. Given the apparent speed of the test and the number of files to invilved, approximately 24K files per hour, this is going to take MANY HOURS. I'll check
back when chkdsk has run.

Start your Windows in Safe Mode. If this makes the problem go away and you are able to carry out your operation then it indicates that some third-party application may be interfering and causing the issue.

Restart your computer in Clean Boot State and try to identify the offending item,
and then disable or remove it.

If the above didn't help, disable your antivirus software temporarily and see if it makes the problem go away. Some security software are known to cause these issues sometimes.

Restarted in safe mode. I could not get backup to even start. It's the first option shown in the search box but clicking on it does nothing, as if I'm being ignored.

Restarted in normal mode. Temporarily disabled AV (ESET) and started backup. While watching progress meter I noticed the backup aborts a little more than half way through. AV was disabled for ten minutes. Trying again with AV disabled for one hour.

If the backup still fails, and you receive the 0x8007045D error at the very end of the backup process when it is around 99% complete, you can refer to this
article
and follow the advanced troubleshooting.